The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
The invention is particularly applicable to fuel injection systems for injecting either liquid or gas. Accordingly, the invention will primarily be discussed in relation to operation of an engine on a combination of a liquid fuel and a gaseous fuel, and optionally also on either of the two fuels separately.
The liquid fuels may, for example, comprise ethanol and diesel, and the gaseous fuels may, for example, include hydrogen and CNG, as well as mixtures of CNG and hydrogen.
Fuel injection systems for injecting either liquid or gas are known.
For greater flexibility in the use of fuel, it is becoming desirable to be able to inject fluid comprising a liquid or a gas or a mixture thereof.
There have been various proposals for enabling delivery of two different fuels to an engine.
One such proposal is disclosed in WO 2010/089568 which is directed to controlling the supply of a first fuel and a second fuel to an engine which is fuelled by the first fuel only in a first mode of operation and by a mixture of the first and second fuels in a second mode of operation. The arrangement involves supplying a reduced amount of the first fuel and supplying a substitute amount of the second fuel to compensate for the reduction in the amount of the first fuel to provide a predetermined combined air to fuel ratio for each power stroke of the engine. The quantities of first and second fuels are metered separately and also delivered through separate fuel injectors; that is, the two fuels are not brought together and delivered in a common injection event through the same delivery port.
Another proposal is disclosed in EP 1 496 247 which is directed to a fuel delivery system for delivering a primary fuel such as diesel fuel to a fuel injector, and a second fuel delivery system for concurrently delivering a second fuel to the engine. The amount of the first fuel delivered to the engine is determined so that the correct amount of secondary fuel can be delivered. The secondary fuel is delivered concurrently with the first fuel but as a separate action; that is, the two fuels are not brought together and delivered in a common injection event through the same delivery port.
For certain applications, it is advantageous to effect injection of the two fuels through a common injection event; specifically, through a single injection device and via the same delivery port.
One proposal which can deliver two separate fuels in a common injection event is disclosed in US2002/0195088 which is directed to a dual fuel system for feeding a main fuel and a secondary fuel to an internal combustion engine. The dual fuel system utilises a common fuel injector connected to two separate fuel circuits, one for the main fuel and the other for the secondary fuel. Each fuel circuit has a separate fuel rail to which the common fuel injector is connected. The fuel injector has two separate fuel paths isolated from each other and each communicating with a respective fuel rail. As the fuel injector is configured to provide two separate fuel paths, the two fuels can be delivered in a common injection event. However, the two fuels are metered separately of each other and are not brought together until delivery; that is, the two fluids are not delivered in unison through the same delivery port. Accordingly, there is no requirement for metering of one fuel in the presence of the other, which would likely be required if the two fuels involve both liquid and gas components to be delivered in a common injection event through the same delivery port.
Where the two fuels are injected in a common injection event and involve both liquid and gas components, there can be a need to vary the relative proportions of the two components or compensate the amount of one due to the presence of the other.
Any variation in the quantity of gas delivered with the liquid may, however, present challenges in certain applications. The is because a variation in the quantity of one of the fuels necessitates a counterpart variation in the quantity of the other fuel. Furthermore, when a liquid fluid is injected concurrently with a gaseous fluid during the metering event for the gaseous fluid, this may change the characteristic flow rate for the gaseous fluid which is a fundamental input to the control of the metered quantity of the gaseous fluid.
Such challenges increase in circumstances where there is a requirement for an engine to run on a combination of the gaseous fuel and the liquid fuel, and optionally also to run on either of the fuels separately.
It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.